Nanocrystalline TiO₂-based mesoporous electrodes are frequently used in DSSCs because TiO₂ exhibits the highest efficiency among the investigated materials, which also include ZnO, SnO₂, Nb₂O₅, SrTiO₃, Zn₂SnO₄, and ZnO-coated SnO₂ (Hagfeldt et al., 2010). In an efficient DSSC, the photoelectrode should have the following characteristics (Hasan et al., 2020):

The development of efficient WEs based on various types of materials is accordingly reviewed in this section. A modified photoelectrode was developed by adding nitrogen heterocycles as electron-donors, which produced a negative shift in the conduction band and reduced electron recombination (Lau and Soroush, 2019). Other additives in the semiconductor materials with a suitable cation/anion have also sufficient conduction band edge providing high electron injection rate (Hasan et al., 2020). A DSSC-based metal oxide semiconductor co-adsorbed with guanidinium [C(NH₂)3+] cations achieved an efficiency of 11.04% (Grätzel, 2004). Furthermore, adding other cation dopants, such as Ni, W, Al, Nb, Sn, Li, Fe, Cu, Zn, Ce, La, and Eu, to semiconductor materials can change by the interfacial energy levels leading to faster electron hole separation from dye to mesoporous electrodes and also reduced the recombination of photoelectrons at the electrolyte–semiconductor interface (Watson and Meyer, 2004; Roose et al., 2015; Hasan et al., 2020). The anionic dopants, including N, B, C, S, P, F, I, and Br, have also been reported (Zhang et al., 2010; Cheng et al., 2015). In comparison to cationic dopants, anionic dopants have a greater electrical conductivity because of their superior thermal and photochemical stabilities (Hasan et al., 2020).

Besides cationic/anionic additives, Jarernboon et al. (2009a) studied the effects of multiwall carbon nanotubes (MWCNTs) on the reduction of the microcrack formation in metal oxide TiO₂ films. They successfully synthesized a nanocomposite TiO₂/MWCNT film using an electrophoretic deposition technique, finding that a minimal addition of MWCNTs to the TiO₂ solution significantly reduced the degree of TiO₂ cracking. It was suggested that MWCNTs minimize cracking by bonding to TiO₂ particles through the interaction of their hydroxyl and carboxylic groups. The spectrophotometric measurements revealed that the MWCNTs were well dispersed on the film, and the amount of MWCNTs incorporated on the film increased with increasing MWCNT concentration in the solution.

Jarernboon et al. (2009b) reported the optimization of TiO₂ films for DSSC applications prepared via electrophoretic deposition. It was found that the thickness of the TiO₂ films increased proportionally with the deposition time and voltage. However, as the deposition time or deposition voltage increased, the film surface became more discontinuous and microcracks became evident. The characteristics of these films as WEs were also analyzed. The energy conversion efficiency and photocurrent density were both dependent on TiO₂ thickness. The curve fitting of the relationship between energy conversion efficiency and TiO₂ thickness revealed an optimum solar cell efficiency of ~2.8% at a film thickness of ~14 μm.

Saekow et al. (2012) reported a huge increase in the photocurrent density and conversion efficiency of DSSCs manufactured using a high-intensity UV-O₃ treatment on TiO₂-layered WEs. This high-intensity UV-O₃ treatment reduces the charge-transfer resistance and increases the amount of dye adsorbed onto the TiO₂ surface. This appears to have the same effect as the sintering of electrical contacts through UV illumination during the manufacturing of the semiconductors.

Photosensitizers are a critical component of DSSCs that play an essential role in light absorption and electron injection into the conduction band of a semiconductor. A dye sensitizer acts to absorb photons that contain energy equal to or higher than the energy gap of its molecules, improving the efficiency of DSSCs. Indeed, a sufficient driving force is required to convert light into electricity (Sang-aroon et al., 2019), providing an opportunity to design and develop molecular dyes for use as photosensitizers in DSSCs. Such dyes can either be derived from natural sources or be synthesized, and their molecular structures have been improved considerably by ongoing research. The molecular architecture of an effective photosensitizer has the following essential characteristics (Hagfeldt et al., 2010):

Many different sensitizing dyes, including synthetic dyes (i.e., metal complexes and metal-free organic dyes) and natural dye pigments, have been evaluated for the application as photosensitizers in DSSCs according to the requirements in the studies by Sang-aroon et al. (2012, 2013, 2014), Chaiamornnugool et al. (2017), Tontapha et al. (2017), and Phinjaturus et al. (2016). The theoretical calculations are an important tool that can be used to understand, express, predict, and optimize the properties of the photochemical mechanism of sensitizing dyes during the intramolecular transition. In particular, a fundamental understanding of density functional theory (DFT) is helpful for designing and developing molecular structures to achieve better photosensitizer efficiency. Therefore, the DFT and time-dependent DFT (TD-DFT) methods have been used to calculate and predict the geometric structures of different dyes and explain their interactions within DSSC cells at an electron scale (Sang-aroon et al., 2012, 2013, 2014; Chaiamornnugool et al., 2017; Tontapha et al., 2017).

Among the natural sensitizing dyes, monascus (Sang-aroon et al., 2012), cochineal and lac dyes (Sang-aroon et al., 2013), orcein dyes (Sang-aroon et al., 2014), anthocyanins (Maiaugree et al., 2015a; Phinjaturus et al., 2016; Chaiamornnugool et al., 2017; Tontapha et al., 2017), mangosteen dyes (Maiaugree et al., 2015a; Tontapha et al., 2017), and others (Sang-aroon et al., 2019) have shown good photosensitizing properties in both theoretical and experimental procedures. The main properties of DSSCs with various dye sensitizers are summarized in Table 1. However, the conversion efficiency of natural dyes when used as molecular sensitizers is still lower than that of synthetic dyes. Thus, it is necessary to search for new and highly effective synthetic dyes for the development of more effective photosensitive materials. Alternatively, the dye adsorption patterns can be designed with double- or triple-layered architectures to provide high-efficiency DSSCs. However, this is currently a challenge, as molecular engineering must be used to modify the electronic and molecular structures of the dyes to shift their absorption into the visible light region. Additionally, the adsorption kinetics of sensitizing dyes should be experimentally determined to develop an understanding of the adsorption rate of individual dyes.

Allotropes of carbon have previously been reported as excellent catalysts because of their low cost, large surface area, high corrosion resistance toward iodine, high reactivity for tri-iodide reduction, and suitable chemical compatibility with other elements (Grätzel, 2003). The use of magnesium nanoparticles and molecular dopants such as aniline (AN) and nitrobenzene (NB) on carbon nanostructures can provide high-performance catalysts for DSSCs based on iodide and cobalt-based electrolytes (Pimanpang et al., 2009; Rakspun et al., 2020). Furthermore, mixed powders containing MWCNTs and cement were coated onto a fluorine-doped tin oxide (FTO) substrate at a 29:71 ratio using a simple painting method without heating, resulting in a greater efficiency (9.60%) than that of a pure carbon DSSC (4.99%) (Chindapasirt et al., 2019).

An annealing treatment is commonly proposed to improve the electrical and electrocatalytic activities of DSSC materials. Siriroj et al. (2012) reported that hydrothermally depositing MWCNT onto a CE annealed at 180°C improved the efficiency of the DSSC from 2.37% (non-annealed) to 7.66%. Wang et al. (2011) reported that a DSSC using N-doped resistive diamond-like carbon (DLC) films as a catalyst exhibited an efficiency of only 1.57%; however, Park and Kim (2011) reported that the efficiency of a DSSC with a DLC film increased to 5% when using a 600°C annealing treatment on the DLC film. Similarly, the diamond-like and graphite-like composited films obtained by annealing a hydrogenated DLC film have been applied as efficient catalysts (Uppachai, 2015); annealing a DLC film at 500°C increased DSSC efficiency from 0.24 to 7.61%.

Nanoporous carbon microspheres (CMS) have been synthesized using a hydrothermal deposition method with carrot juice as the natural carbon source. An annealing process was then performed to increase the conductivity and surface area (149.10 m²/g) of the resulting microspheres. The efficiency of the cell using this CMS CE was 0.17% before annealing and 7.71% after annealing (Lowpa et al., 2015). Maiaugree et al. (2015a) synthesized a mangosteen peel carbon (MPC) using a sintering process and applied it as a catalyst with a large surface area of 125 m²/g, in a DSSC comprising a T₂/T⁻ electrolyte. The efficiency of this DSSC reached 2.63%, which was higher than that of a DSSC with a Pt CE (1.47%).

The electrocatalytic activities of the T₂/T⁻ electrolyte and non-metal catalysts have also been investigated. Tangtrakarn et al. (2019) synthesized a post-annealed evaporated-carbon CE for a DSSC with a T₂/T⁻ electrolyte, achieving a solar efficiency of 8.04%. A short-term stability test was also performed over 50 days; the efficiency of the carbon DSSC was observed to remain constant, whereas that of the Pt DSSC decreased by 26%.

A cofunctioning catalyst was proposed to further enhance the electrocatalytic activity and surface area of the CE, resulting in improved solar cell performance. Enhanced DSSC performance was obtained by Towannang et al. (2012) and Maiaugree et al. (2012a) by using a polypyrrole (PPy) cocatalyst mixed with MWCNTs and MWCNTs/Pt, respectively. Similarly, the efficiency of a DSSC using an I3-/I⁻ or T₂/T⁻ electrolyte was found to increase with the addition of the nanoparticle composites of cobalt or nickel to N-doped mesoporous carbon (Hasin et al., 2017). A pronounced 3.59% improvement in performance, close to that of a Pt DSSC (3.84%), was achieved by compositing TiC with carbon as the catalyst (Towannang et al., 2012). Furthermore, a combination of chemical bath deposition, an arc evaporation process, and annealing was used to synthesize a bilayered carbon-decorated Ni₃S₂ composite nanowall film as a cocatalyst (Maiaugree et al., 2015b). The large surface area of the nanostructure and its high electrocatalytic activity improved the efficiency of a pure evaporated-carbon DSSC. The increased active surface area of nickel sulfide (Ni₃S₂) nanoparticles coated on MWCNTs prepared using a hydrothermal deposition process was also found to enhance DSSC performance (Maiaugree et al., 2019).

An advantage of metal-based and composite CE cocatalyst materials is that they can be used in DSSCs instead of a pure PPy or PEDOT:PSS conductive polymer (Wu et al., 2008; Keothongkham et al., 2012). The composite catalyst materials, such as PEDOT-PSS+TiO₂ (Maiaugree et al., 2012b), NiSO₄+PEDOT:PSS (Maiaugree et al., 2013), NiS+PEDOT:PSS (Maiaugree et al., 2017), and SrTi_1−xO₃+PEDOT-PSS (Maiaugree et al., 2018), have been prepared by using various methods employing doctor-blade applications, electrodeposition, or hydrothermal deposition processes. The efficiencies of DSSCs employing these composite materials were found to be significantly increased because multiple catalysts were excited. Towannang et al. (2018) used a hydrothermal process to fabricate a tungsten carbide (WC) CE catalyst with Pt or Pd particles for DSSCs with a T₂/T⁻ electrolyte, resulting in considerably improved cell efficiency. Maiaugree et al. (2014) synthesized the connected networks of sputtered Pt on porous nickel oxide (PNO) nanosheets for use as a catalyst. The solar conversion efficiency of the FTO/PNO/Pt DSSC was 8.17% compared to the 7.23% efficiency of the FTO/Pt DSSC, as the electrocatalytic activities were enhanced and the electroactive areas were increased. A tungsten oxide (WO_3−x) CE was easily prepared via thermal annealing of a tungsten sheet under a low-pressure O₂ atmosphere (Uppachai et al., 2014); the resulting presence of W⁶⁺, W⁵⁺, and W⁴⁺ valence states in the sub-stoichiometric WO_3−x produced excellent catalytic activity and electrical conductivity.